Primers are often used as an undercoat in combination with a covercoat adhesive in order to achieve superior bonding between two substrates made from different materials. One particular application for such primers is in bonding elastomers to metal surfaces. Elastomer-to-metal bonding is subjected to severe environmental conditions in many industrial and automotive assemblies. For example, many engine mounting assemblies that employ elastomer-to-metal bonding contain fluids in order to assist in damping of vibration of the engine. These fluid-filled engine mounting devices are being exposed to increasingly high temperatures such that the elastomer-to-metal adhesive bonds within the mounts are being exposed to very high temperature fluid environments. Many elastomer-to-metal assemblies, particularly those utilized in automobile applications, are routinely exposed to materials that contain corrosive salts or other corrosive materials that may act to degrade the elastomer-to-metal adhesive bond.
In order to provide acceptable bonding, adhesive compositions must exhibit excellent primary bonding defined as retention of rubber on the substrate after bond destruction, adequate sweep resistance i.e., ability of the uncured adhesive coating on the substrate to remain undisturbed against the force of injected green rubber into a mold cavity, good storage stability of the adhesive and durable adhesion under extreme environmental conditions, typically measured by the hot tear test (such as ASTM D-429) the stressed boiling water test (well known in the industry), and salt spray tests (such as ASTM B-117-97).
In light of the increasing regulations regarding volatile organic compounds (VOC), the use of traditional solvent-borne adhesives is becoming more problematic. Consequently, there is significant ongoing work to develop water-borne replacements. For example, aqueous or water-borne primers are known and described in U.S. Pat. Nos. 5,200,455 and 5,162,156, but they have various shortcomings compared to solvent-borne primers. For example, it is desirable to improve the environmental resistance performance of aqueous elastomer-to-metal adhesion primers that include polyvinyl alcohol-stabilized phenolic resin dispersions (see U.S. Pat. No. 5,200,455) when used with certain important adhesive covercoats. Elastomer-to-metal primers that include phenolic resins derived from water soluble phenolic precursors are also known (see U.S. Pat. No. 5,162,156) but these suffer from drawbacks that prevent their use to form robust, environmentally resistant films. Before being thermoset, films formed from water soluble phenolic resins tend to re-solvate when exposed to water. Often this source of water is an aqueous covercoat applied to the primer film. Application of the aqueous covercoat essentially washes away the film formed from the water soluble phenolic resin. In addition, such films exhibit very limited resistance to corrosive fluids.
Another problem associated with the bonding of elastomer to metal relates to pre-heating or pre-baking of the dried adhesive prior to bonding. The metal substrate typically is coated with the adhesive, the adhesive is dried and then the adhesive-coated metal substrate is placed in a mold. Elastomer then is introduced into the mold and bonded to the metal substrate during vulcanization of the elastomer. The bonded part is removed from the mold and the next metal substrate is placed in the mold. When this subsequent metal substrate is placed in the mold it is subjected to the heat retained in the mold from the previous molding operation. The dried adhesive must be able to withstand this retained heat (referred to herein as “pre-heating”) prior to bonding.
Consequently, there exists a continuing need for a primer that provides robust adhesive bonding in hot, corrosive environments, has an affinity for a broad range of covercoats, and forms a flexible film that is resistant to pre-heating conditions and exhibits superior shelf-life stability and resistance to resolvation. It would be especially advantageous if such a primer could also be used as a coating for protecting a metallic surface.